Digital printing apparatus using liquid toner are known from US Patent Application Publication No. 2011/0249990. The known digital printing apparatus comprises a feed roller, a developing roller, developing roller cleaning means, and an image carrying roller; the feed roller being arranged to transfer a quantity of liquid toner from a reservoir onto the developing roller; and the developing roller being arranged to transfer a portion of the quantity of liquid toner onto the image carrying roller in accordance with a charge pattern sustained on a surface of the image carrying roller.
In digital printing systems of this kind, it is necessary to remove the liquid toner residue that remains on the surface of the developing roller after contact with the imaging roller (typically a roller with a photoconductive surface, adapted to carry a latent image formed by a pattern of charges on that surface). The removal of this residue is quite challenging.
European Patent Application Publication No. 2685320 in the name of the Applicant describes the use of an oscillating electric field arranged to substantially decompactify the chargeable imaging particles in a liquid toner residue on a developing roller, prior to or during its mechanical removal.
In the digital printing apparatus according to the aforementioned US Patent Application Publication No. 2011/0249990, an upstream corona charger is arranged opposite to a surface of the developing roller downstream of the area of its rotational contact with the feed roller and upstream of the area of its rotational contact with the image carrying roller, and a downstream corona discharger is arranged opposite to a surface of the developing roller downstream of the area of its rotational contact with the image carrying roller and upstream of the area of its rotational contact with the developing roller cleaning means. The downstream corona discharger applies charges which are of a polarity opposite to that of charges applied by the upstream corona charger to impart a force to toner particles that are flocculated or agglutinated to remain on the surface of the developing roller by the upstream corona charger. It is alleged that the resulting force acts in a direction in which the toner particles come off the surface of the developing roller, and it is claimed that this results in easier removal of the residual toner from the surface of the developing roller.
It has been found, however, that the arrangement of US Patent Application Publication No. 2011/0249990 does not always lead to optimal removal of the residual liquid toner.